By the end of last year U.S. primary aluminium output was running at an annualised rate of 1.15 million tonnes, equivalent to 63 percent of domestic capacity, according to figures from the Aluminum Association.

It should rise further as Century Aluminum reactivates a third idled line at its Hawesville smelter in Kentucky. The first line kicked back into life in the third quarter and the second was due by the end of last year.

Century is one of three companies actively rekindling U.S. production.

Magnitude 7 Metals has resumed operations at the New Madrid smelter in Missouri. The import tariff has certainly helped but the restart plans were underway as soon as the plant was bought in 2016.

Similarly, Alcoa's restart of idled capacity at its Warrick smelter in Indiana was announced at the end of 2017, before Commerce had submitted its report, and was driven, the company has since stressed, by plant-specific economics not tariffs.

It remains to be seen how much more dormant capacity can be coaxed back into life because the economics of smelting aluminium remain challenging.

Alcoa reported a Q4 2018 operating loss in its aluminium segment and the price of the metal has sunk further since then. Trading at $1,860 per tonne on the London Metal Exchange Tuesday morning, aluminium has failed to bounce much from January's one-year low of $1,785.50.

Ask just about any analyst what's going on with the bombed-out price and you'll be pointed in the same direction.

China's exports, mainly of aluminium in semi-manufactured product form ("semis"), surged 21 percent to 5.8 million tonnes last year.

They jumped again to 552,000 tonnes in January, the growth rate accelerating to 26 percent.

RISING EXCLUSIONS

China has for several years been the dominant source of U.S. imports of aluminium "semis".

There are anecdotal reports that Chinese exporters are shipping more to other Asian countries and less to the United States, partly due to antidumping duties on specific products and partly due to the broader U.S.-China trade tensions.

There are another 590 exclusion requests for Chinese products pending.

Canada, by contrast, has garnered just 3 exclusions, covering less than 5,000 tonnes, although there were 935 requests still pending as of mid-December.

The comparison between the fortunes of the United States' two biggest aluminium suppliers is slightly misleading but still instructive.

Canada has historically been the dominant supplier of primary aluminium to the United States, although it ships "semis" as well.

It's hard to argue that metal in this raw form can't be sourced domestically. The second and third largest suppliers of primary aluminium in 2017 were Russia and the United Arab Emirates and neither has received any exclusions either.

China doesn't export that sort of aluminium but rather a broad spectrum of "semis", including, evidently, many that can't be sourced in the domestic market.

It shouldn't be surprising that the exclusion mechanism favours product over metal.

But it does expose the limitations of tariffs when the U.S. remains so dependent on imports.

That includes primary metal. Even if all the idled capacity in the country were restarted, the United States could still meet less than half of its primary needs, according to the Aluminum Association.

PEAK EFFECTIVENESS

There were just over 6,200 aluminium exclusion requests pending as of Dec. 18, according to the Mercatus Center. The Commerce Department is also having to deal with 30,000 requests for exclusion from the steel tariffs.

Each one represents another potential hole in President Donald Trump's aluminium wall.

True, domestic production is creaking back into life but the 10 percent tariff can provide only limited cushioning for U.S. smelters as the price deteriorates.

The reason these plants were mothballed in the first place was because of their higher cost structures.

Tariffs may already be close to, if not past, peak effectiveness from the administration's stated national security perspective.

What happens next then?

Aluminium associations the world over are calling for a refocus on China's massive production and export capacity.

The OECD has provided them with ammunition by concluding in a recent report that "market distortions appear to be a genuine concern in the aluminium industry". It cited in particular China and the Gulf Cooperation Countries. ("Measuring distortions in international markets: the aluminium value chain", January 2019)

Maybe the U.S. administration is going it alone and aluminium is part of the broader trade talks about Chinese subsidies.

If it wanted, though, it would find plenty of allies for a multilateral push for reform of the Chinese aluminium sector.

Mind you, it would probably have to start by making peace with the Canadians.